Anadromy in Fragmented Rivers: Downstream Passage Challenges for Migrating Salmonids

Migration is essential for anadromous salmonids; however, hydropower electric power (HEP) development has fragmented rivers and disrupted downstream passage for juveniles (smolts) and adults (kelts), delaying migration and increasing mortality. Although passage solutions can mitigate these impacts, their performance remains rarely evaluated, particularly in river systems with multiple barriers. This thesis assessed downstream passage performance in fragmented rivers for smolts and kelts, identified factors influencing passage, and evaluated mitigation effectiveness. Acoustic telemetry quantified movement patterns, route choice, delays, migration speeds, passage and loss rates in rivers containing multiple HEPs across large- and small-scale systems. Study sites included HEPs with and without downstream passage solutions and freeflowing river sections. Free-flowing sections exhibited some of the highest passage rates, and HEP removal more than doubled migration speeds. Passage losses and reduced migration speeds were highest in lentic habitats and HEP sections, identifying these areas as bottlenecks. In contrast, HEPs with dedicated downstream passage solutions showed improved performance, with faster migration speeds and lower loss rates. Water temperature, diel period, body size, and river discharge influenced smolt passage success, with flow velocity strongly associated with movement behavior. Control release groups identifying cumulative barriers possibly influence passage speeds, and in-river overwintering by kelts influenced passage success. Overall, while passage efficiency can be high at individual HEPs, river-scale migration remains constrained by interacting natural and anthropogenic factors. Downstream passage solutions improved connectivity and reduced delays, highlighting the importance of site-specific mitigation measures for sustaining anadromous salmonid populations in fragmented rivers.